1. Field of the Invention
The present invention relates to an arc ion plating apparatus improved in stability of metal ion bombardment.
2. Description of the Related Art
In recent years, hard film deposition (TiN, TiAlN, CrN, etc.) by PVD has been performed to a substrate (film deposition object) for the purpose of improvement in wear resistance of a cutting tool or improvement in tribological characteristic of a sliding surface of a mechanical part. The industrial technique frequently used for such a hard film deposition is arc ion plating (hereinafter referred to as “AIP”) which evaporates a film deposition material by vacuum arc discharge to form a film on a substrate surface, and an apparatus for performing such a film deposition is called an arc ion plating apparatus (hereinafter referred to as “AIP apparatus”).
The AIP apparatus comprises, as shown in FIG. 10, a vacuum chamber 1, and a rotary table 2 arranged on the bottom of the vacuum chamber 1 so that the table upper surface is horizontal. The rotary table 2 is rotated by a rotating shaft 3, and a plurality of planetary shafts 4 protruded from the upper surface of the rotary table 2 also rotate around their own axis by a planetary gear mechanism provided within the rotary table 2. A substrate holder 5 for holding a substrate is detachably mounted on each planetary shaft 4. Therefore, each substrate holder 5 rotates around its own axis while moving horizontally by the rotation of the rotary table 2, and the substrate such as a tool, a die or a mechanical part held by this substrate holder 5 rotates around its own axis by the rotation of the substrate holder 5 while revolving by the rotation of the rotary table 2. Negative voltage is applied to the rotary table 2 by a bias power supply (not shown), and this negative voltage is applied, through the substrate holder 5, to the substrate loaded thereon.
An arc evaporation source for deposition group 7 composed of three evaporation sources 7A arranged in line at substantially fixed intervals in the height direction of the vacuum chamber 1 is provided on the side wall inner surface of the vacuum chamber 1, and the evaporation sources 7A are connected to negative electrodes of arc power supplies 8, respectively, with the positive electrodes thereof being connected to the vacuum chamber 1. In FIG. 10(B), denoted at 21 is a pumping port for evacuating the vacuum chamber, 22 is a gas supply pipe for supplying a process gas such as nitrogen or oxygen (omitted in FIG. 10(A)), and 23 is an opening and closing door of the vacuum chamber.
A procedure for forming a functional film on the surface of a substrate using the AIP apparatus will be briefly described. The substrate is loaded on the substrate holder 5 and set on the rotary table 2, the vacuum chamber 1 is evacuated, the substrate is heated by a heater (not shown) provided within the vacuum chamber 1, and metal ion bombardment (hereinafter often simply referred to as “bombardment”) is then performed to improve the adhesion of the film to be formed. The bombardment is a process for irradiating the substrate applied with a minus voltage of not less than several hundreds V (generally, 600 to 1000V) with metal ions evaporated from the evaporation sources 7A to etch the surface layer of the substrate by high-energy ion irradiation or to form a mixed layer of irradiation ions and the substrate.
After the end of bombardment, vapor of metal ions is generated from the evaporation sources 7A and irradiated to the substrate, and the voltage to be applied to the substrate is set to about 0 to −300V, whereby film deposition is started. Since a film to be formed by the AIP generally consists of a compound of a metal such as TiN, TiCN, CrN, TiAlN, TiC, or CrON with nitrogen, carbon, oxygen or the like, process gases such as nitrogen, oxygen, and hydrocarbon are introduced into the vacuum chamber 1 singly or in combination thereof during film deposition. For example, introduction of nitrogen with evaporation of Ti results in film deposition of TiN (titanium nitride).
Since the substrate loaded on the substrate holder 5 performs revolution and rotation by rotation of the rotary table 2 in the bombardment and the film deposition, uniform ion irradiation can be performed to the whole substrate.
After film deposition, cooling is performed, the vacuum chamber 1 is opened, and the substrate with a film formed thereon is taken out with the substrate holder 5 to recover the film-formed substrate after film deposition.
While the above-mentioned AIP apparatus performs the bombardment and the functional film deposition by use of the arc evaporation source for deposition group 7, Japanese Patent Laid-Open No. Hei 4-276062 discloses an AIP apparatus comprising an arc evaporation source for deposition and an arc evaporation source for bombardment of the same shape as this, which are provided within a vacuum chamber. According to this apparatus, since high-melting point metal or high-mass metal can be used as the evaporating material for arc evaporation source for bombardment even in use of low-melting point metal (e.g., TiAl alloy) as the evaporating material for arc evaporation source for deposition, the problem that the low-melting point metal disables effective bombardment treatment because of its reduced ionization ratio, and the problem of the deposition of droplets to the substrate surface can be solved.
It is known that there is a minimum arc current in order to stably operate the arc evaporation source for deposition and the arc evaporation source for bombardment, regardless of the size of evaporating surface. This minimum current is varied depending on the evaporating material and gas atmosphere. When a material such as Ti or TiAl alloy is used as the evaporating material for hard film deposition, generally, a current of at least about 80 A is needed in an environment to which gas is hardly introduced or in an environment for performing bombardment, and a current value smaller than this makes arc discharge unstable. In the bombardment process, metal ions are generated from an evaporation source in a state where a negative voltage of not less than several hundreds V (in general, about −600 to −1000V) is applied to the substrate. However, since the lower limit of arc current to evaporation source is defined for stable operation as described above, the irradiation quantity of metal ions also inevitably reaches a certain quantity.
Therefore, there is the following problem in the bombardment. Stable arc discharge requires an increased energy input quantity to substrate even with a minimum current value and, particularly, in a substrate with small heat capacity such as a drill with small diameter, the substrate temperature is rapidly raised. In order to prevent such an over-temperature rise, process conditions must be controlled in a short-time unit, such that the bombardment time is set to a short time to repeat bombardment with cooling interval. Therefore, the controllability is poor, and productivity is consequently reduced.
Further, as the arc evaporation source, two or more evaporation sources of relatively small size with a diameter of about 50 to 180 mm, typically with a diameter of about 100 to 150 mm are frequently used. However, since simultaneous operation of a number of evaporation sources requires a bias power supply of large capacity, and causes irradiation-off of a large quantity of metal ions, the problem of frequent occurrence of abnormal discharge on substrate is caused, in addition to the over-temperature rise of substrate. Since the bias power supply temporarily stops output in the event of abnormal discharge, an accurate bombardment process cannot be executed if abnormal discharge frequently occurs in course of short-time bombardment.